A charged particle detector is the indispensable part of a charged particle (ion or electron beam) instrument, such as a scanning electron microscope (SEM). In a SEM, an electron beam emanated from an electron source is focused into a fine probe over a specimen surface and scanned by a deflection unit in a raster fashion; and signal electrons released from the specimen, including secondary electrons and back scattered electrons, are collected by charged particle detectors and the signal intensity is converted into the gray level of an image pixel corresponding to the location of the electron probe on the specimen surface. Scanning of the electron probe will then form a gray level mapping for producing the image of the specimen surface. A low voltage SEM, in which an incident electron beam has the energy of 3 keV or less, is known to be particularly effective in evaluating topographic features of specimen surface due to the dominance of secondary electrons in the signal electrons. Secondary electrons are originated within a shallow depth from the specimen surface; their yield and trajectory are influenced by the surface topography and thus carry the topographic information.
The most common detectors used in SEM are of the scintillator-photomultiplier tube (PMT) combination type (such as an Everhart-Thornley detector), the semiconductor type, and the microchannel plate (MCP) type. The scintillator-PMT type detector, due to high gain and low noise properties, is more frequently used in high resolution SEM in which the beam current is low. Furthermore, the scintillator-PMT type detector generally comprises a light guide rod that has a front face coated with a light-generating scintillator and is coupled to a photomultiplier tube to form a unit. A common arrangement is to position one or a multiple of these units below the final focusing objective lens, surrounding the impact point of the primary electron beam, with the front face covered with a positively biased grid to attract the secondary electrons emitted from the specimen in what amounts to a side detection scheme. FIG. 1 is a schematic diagram illustrating the fundamental structures of a conventional SEM. A side detector 108 is inserted between an objective lens 103 and a specimen 104. Recently, increased demand on low voltage SEMs of higher resolution has prompted more widespread use of SEMs with an immersion type of objective lens for its ability to provide finer electron probes due to smaller electron optical aberrations. In a SEM with an immersion type of objective lens, the specimen is immersed in the strong magnetic focusing field of the objective lens, over which an electrostatic extraction field is also typically superimposed. While the main purpose is to focus the primary electron beam, the magnetic field also confines the secondary electron trajectories close to the central optical axis, with the electrostatic field acting to pull the electrons away from the specimen 104 into the center bore of the objective lens 103. In this case, the side detector 108 can no longer receive any secondary electrons, and in-lens detectors must be used instead. The side detector has the advantage of detecting sample three dimensional topographic information and the in-lens detector has the advantage of high detection efficiency. In order to detecting more sample topographic information, the present invention proposes a multiple channel in-lens detection system, in which the multiple channels are annular systematically distributed around the primary beam, forming an on-axis detection system.
Multi-channel detection system can be used for enhancing topographic features on the specimen surface without tilting the specimen. The detector in such a detection system is divided into equal halves or quarters from which signal outputs are respectively processed and displayed separately. Such idea is often used in off-lens detection system. As illustrated in FIG. 1, a segmented front lens Robinson detector 120 is positioned under the lower face of the objective lens 103 and faces the specimen 104 for receiving mainly backscattering electrons. Each of the segments of the detector 120 has independent channels to form images. The signals from the channels of the detector 120 are processed, including addition or subtraction, to provide large quantity of information of the specimen 104 including topographic and material information.
Compared to an off-axis detector, on-axis in-lens detector benefits the system resolution. Off-axis in-lens detector needs a Wien filter to band the secondary electron off axis and guide them to the detector. Banding secondary electron off axis introduces the second order aberration for a primary beam and lowers the resolution of a system.
The annular and symmetric arrangement of the multiple channels addresses the three dimensional topographic imaging of a sample surface. As illustrated in FIG. 2, secondary electrons 205B emanated from the side surface 204L of a surface feature 208 strike a detector half 206B, while another secondary electrons 205A from the side surface 204R strike another detector half 206A, after crossing the optical axis under the focusing effect of the magnetic field from the objective lens 203. When the signals respectively from the halves are added together as in a non-segmented detector case, both edges of the surface feature 208 will demonstrate bright in the image (as illustrated in the line profile 2A) as more secondary electrons 205A or 205B are emitted along the side wall region, making it difficult to discern whether the surface feature 208 is a protrusion or a depression. However, when the signals from the halves are displayed individually, a shadow effect is directed to one bright edge of the surface feature 208 compared to the dark one, as illustrated in the line profile 2B from the detector half 206A and the line profile 2C from the detector half 206B. Thus, a three-dimensional impression is generated and directed to the protrusion of the surface feature 208. In the case of a two-channel detector 109, as illustrated in FIG. 1, sandwiching the primary beam, the shadow effect may be formed in the direction perpendicular to the two-channel detector 109 rather than parallel to the two-channel detector 109. Thus, three-dimension sample surface information can be distorted on the image obtained from the two-channel detector 109.
An alternative detector configuration potential to overcome this problem, an annular in-lens on-axis symmetrically distributed multi-channel detection system has, however, never been made or reported, for example the in-lens segmented multi-channel on-axis Scintillator-PMT type, semiconductor type or micro-plate type of detectors.
Currently, either a single detector is used on axis or multiple detectors are arranged separately around the center optical axis, often working in conjunction with a reflection plate 107 which generates its own secondary electrons to be collected by the detectors, when the plate is struck by the secondary and back-scattered electrons coming from the specimen. These arrangements are schematically illustrated in FIG. 1, with multiple in-lens detectors 106A and 106B positioned off axis in conjunction with a reflection plate 107 the two pieces of detectors 109 is sandwiching the optical axis, and a single Scintillator-PMT detector 105 is positioned on axis. For the multiple detector arrangement the configuration is complicated and the signal collection efficiency is mediocre due to the spatial separation of the individual detectors, while for the single on-axis detector case it is difficult to achieve a uniform signal collection due to the non-rotational-symmetric nature of the light-guide tube.
Accordingly, there is a need in the art for innovative designs for charged particle detectors, so that the high efficiency space saving segmented multi-channel in-lens on-axis annular configuration or its equivalent can be realized for Scintillator-PMT type, semiconductor type or micro-plate type of detectors.